In recent years, there have been more and more researches and developments on how to increase the energy density of a non-aqueous electrolyte secondary battery or a hybrid capacitor. Non-aqueous electrolyte secondary batteries, and the like, are used as power sources for electronic devices such as smartphones, mobile telephones, portable information devices, laptop computers, video cameras and portable game devices, as driving power sources for electric tools, vacuum cleaners, robots, etc., and as power sources for driving or assisting electric motors in electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, fuel-cell vehicles, etc.
One way of increasing the energy density of a non-aqueous electrolyte secondary battery, or the like, is to use a material having a large electrical capacity as an active material of an electrode. For example, where copper fluoride (CuF2) is used as a positive electrode active material and lithium metal as a negative electrode active material, it is possible to obtain an electrical capacity of 528 mAh/g through a reaction as shown in Equation 1. This value is greater than five times the electrical capacitance of Li0.5CoO2, which has now been used as a positive electrode active material of lithium-ion batteries. Note that the potential indicated preceding Equation 1 is a value calculated based on the standard free energies of formation with copper fluoride and lithium fluoride.3.54V:CuF2+2Li++2eCu+2LiF  (1)
Such a reaction is known as a conversion reaction. Where copper chloride (CuCl2) is used as a positive electrode active material, it is theoretically possible to obtain an electrical capacity as high as 399 mAh/g. With copper chloride, the following reactions are possible, as opposed to Equation 1.3.40V:CuCl2+Li++eCuCl+LiCl  (2)3.07V:CuCl2+2Li++2eCu+2LiCl  (3)2.74V:CuCl+Li++eCu+LiCl  (4)
Patent Document No. 1 discloses a non-aqueous electrolyte secondary battery in which copper fluoride or copper chloride is used as a positive electrode active material, and fluorinated solvent is used as the solvent of the non-aqueous electrolyte. Specifically, it is stated that a reaction of Equation 4 occurs following Equation 2, by using trifluoro propylene carbonate (abbreviated as TFPC) as the fluorinated solvent, and using a non-aqueous electrolyte in which lithium hexafluorophosphate (LiPF6) is dissolved at a concentration of 1 M and lithium chloride (LiCl) is dissolved at a concentration of 2.4 mM (specified in Patent Document No. 1 as 100 mg/liter) as electrolyte salts. Now, the reason for using TFPC is to prevent excessive dissolution of copper fluoride or copper chloride in the electrolyte.
Patent Document No. 2 discloses a non-aqueous electrolyte primary battery in which copper fluoride or copper chloride is used as a positive electrode active material. An ionic liquid (specified in Patent Document No. 2 as “ambient temperature molten salt”) made of an organic cation and an anion is used as the solvent of the non-aqueous electrolyte, and an alkali metal ion and a chlorine ion (Cl−) are listed as an example of the cation and the anion, respectively, of the electrolyte salt.
Patent Document No. 3 discloses a non-aqueous electrolyte in which lithium tetrafluoroborate (LiBF4), lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (Li(CF3SO2)2N), or the like, is dissolved as an electrolyte salt in an ionic liquid (specified in Patent Document No. 3 as “ionic liquid”) made of an organic cation and an anion having an alkoxyalkyl group.